This invention relates generally to a system for the transmission and reception of data signals and, more particularly, relates to a method and apparatus for limiting distortions caused by both the transmission medium and the echo path in a multicarrier transceiver.
Multicarrier transceiver design typically requires trade-offs based on sampling rate, bit rate, and symbol rate. Many of the trade-offs are impacted by the ratio of symbol length to cyclic prefix length. A cyclic prefix is typically used to ensure that samples from one symbol do not interfere with the samples of another symbol. The length of the cyclic prefix used is determined by the length of the impulse response of the effective physical channel. However, using a long cyclic prefix is seen to reduce the throughput of the transceiver which is an undesirable result.
Specifically, in discrete multitone transceivers, each symbol is comprised of N time samples, x(n), to be transmitted to the remote receiver plus a cyclic prefix consisting of v time samples. The cyclic prefix is simply the last v samples of the original N samples to be transmitted. The cyclic prefix length is determined by the length of the impulse response of the channel and is chosen to minimize the intersymbol interference. If the impulse response of the channel is of length v+1 or shorter then a cyclic prefix of length v is sufficient to completely eliminate inter-symbol interference. If the impulse response of the channel is longer than v+1 then some inter-symbol interference occurs. Since the efficiency of the transceiver is reduced by a factor of N/(N+v) it is clearly desirable to make v as small as possible.
To minimize the cyclic prefix, as shown in prior art FIG. 1, a small time domain FIR filter, referred to as a shortened impulse response filter (SIRF), is typically inserted in the receiver immediately following the A/D converter. The purpose of this filter is to shorten the impulse response of the effective channel. The effective channel is defined as the combination of the transmit filters, physical channel, receive filters, and the SIRF. Denoting the impulse response of the physical channel and filters h.sub.c (n), the output of the SIRF can be expressed as EQU y(n)=x(n)*(h.sub.c (n)*w(n))
where w(n) is the impulse response of the SIRF and * denotes the convolution operator. It can be shown that if x(n) is transmitted with a cyclic prefix of length v and the effective channel EQU h.sub.eff (n)=h.sub.c (n)*w(n)
is at most of length v+1, that y(n) will only depend upon the current symbol's samples x(n) being transmitted. If the effective channel is not constrained to a length of v+1, then y(n) will depend upon the current symbol's samples x(n) as well as the previous symbol's samples x.sub.p (n). The previous transmit symbol will then contribute inter-symbol interference which decreases the performance of the transceiver.
Regardless of the choice of w(n), it is impossible to shorten the impulse response perfectly as some energy will lie outside the largest v+1 samples of h.sub.eff (n). As a measure of the inter-symbol interference one can measure the shortening signal to noise ratio (SSNR) which is the ratio of the energy in the largest v+1 consecutive samples to the energy in the remaining samples. The largest v+1 consecutive samples will not necessarily start with the first sample. This delay, d, is normally compensated for at the receiver by delaying the start of the receive symbol. The SSNR is defined as: ##EQU1##
An example of a technique for calculating the parameters of the above described SIRF may be seen in U.S. Pat. No. 5,285,474 to Chow et al. which patent is incorporated herein by reference in its entirety. The '474 patent discloses a method for optimizing a set of parameters of an equalizer or SIRF to be used to equalize a multicarrier data signal that has been transmitted through a distorting channel. The method includes the steps of intializing the parameters, repeatedly sending a training sequence through the channel to the receiver, using the equalizer parameters, the received sequence, and a local replica of the training sequence to update a set of channel target response parameters, windowing the channel target response parameters, using the channel target response parameters, the received sequence and the local replica to update the equalizer parameters, and windowing the equalizer parameters. As discussed therein, the training process is repeated until a predetermined convergence condition is detected.
While the disclosed method works for its intended purpose, the method suffers the disadvantage of failing to provide a method for also reducing distortions caused by echo. In particular, known prior art systems, exemplified by the system discussed in the '474 patent, fail to provide a transceiver which has the capability to simultaneously and efficiently minimize the distortions caused by echos. Furthermore, the known methods of calculating equalizer parameters discussed in the prior art are also typically hampered by stability and convergence problems. As such, a need exists to provide a discrete multitone transceiver which incorporates both an improved method for minimizing distortions caused by transmission through a distorting channel and distortions caused by signals traveling through the echo path.
As a result of this existing need, it is an object of the present invention to provide a method for calculating the parameters of a SIRF that will jointly minimize distortions which result from both the distorting channel and the echo path.
It is a further object of the present invention to provide a method for calculating the parameters of a SIRF that is much faster and more assured of correct convergence than any previously known method.
It is another object of the present invention to provide a method which is suitable for on-line implementation.
It is yet another object of the present invention to provide a method which minimizes SIRF coefficient calculating time.
It is still another object of the present invention to develop an algorithm for impulse response shortening that is realizable in off the shelf DSP components.